Week Number Calculator
Find the ISO 8601 week number for any date, or look up the Monday-to-Sunday date range for a given week number and year.
Find the week number for a date
Find the dates for a week number
Quick answer
The ISO 8601 week number tells you which week of the year a date falls in. Weeks start on Monday, and week 1 is the week containing the year's first Thursday. This tool returns the ISO week number and ISO year for any date, and can also show the date range of a given week number — for example, 1 January 2025 falls in ISO week 1 of 2025.
Formula & method
The calculator applies the ISO 8601 rule: shift the date to the Thursday of its week (because week 1 always contains a Thursday), then count weeks from the first Thursday of that ISO year. This correctly assigns late-December and early-January dates, which can belong to a neighbouring ISO year. The reverse lookup finds the Monday that starts the requested week number and shows the seven-day range.
Examples
- Input
- 2025-01-01
- Result
- ISO week 1 of 2025
- Why
- 1 January 2025 is a Wednesday whose week contains the first Thursday of the year, so it is week 1.
- Input
- 2024-12-30
- Result
- ISO week 1 of 2025
- Why
- This Monday begins a week whose Thursday (2 January 2025) lands in 2025, so ISO assigns it to week 1 of 2025, not week 53 of 2024.
- Input
- 2025-07-01
- Result
- ISO week 27 of 2025
- Why
- Counting whole weeks from the first Thursday places 1 July 2025 in week 27.
When to use this tool
- Planning or referencing schedules that use week numbers, common in Europe and in business.
- Aligning reports, sprints, or payroll periods to ISO weeks.
- Looking up the dates that fall within a specific week of the year.
Common mistakes
- Assuming weeks start on Sunday — the ISO standard starts each week on Monday.
- Expecting 1 January to always be week 1; if its week's Thursday lands in the previous year, early-January dates can belong to week 52 or 53 of the prior ISO year.
- Confusing the calendar year with the ISO week-numbering year, which can differ for dates near year-end.
Frequently asked questions
How is the ISO week number defined?
Weeks start on Monday, and week 1 is the week that contains the year's first Thursday — equivalently, the week containing 4 January. This is the ISO 8601 international standard used widely in business and in Europe.
Why can a January date be in week 52 or 53?
If the week containing 1 January has its Thursday in the previous year, ISO assigns those early-January days to the last week of the prior ISO year. The tool shows the correct ISO year alongside the week.
Do all years have 52 weeks?
Most have 52, but some have 53 ISO weeks. A year has 53 weeks when it starts on a Thursday, or on a Wednesday in a leap year.
Does the week start on Sunday or Monday?
The ISO standard this tool uses starts the week on Monday and ends it on Sunday. Some calendars elsewhere start on Sunday, which can give a different week number.
Can I find the dates for a week number?
Yes. Enter a week number and year and the tool shows the Monday-to-Sunday range for that ISO week.
Is anything uploaded?
No. The calculation runs entirely in your browser.
Sources & references
External references open in a new tab. We are independent and not affiliated with these organizations.
- ✓ Free to use
- ✓ No sign-up required
- ✓ Runs entirely in your browser — nothing is uploaded.
- ✓ Formula and method shown above
Provided “as is” for general information only — results may be inaccurate, so verify before you rely on them. No warranty; use at your own risk.
Built and reviewed by HIFreeTools against the formula shown above and any authoritative references cited on this page. See our methodology and editorial standards.
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